A solar cell is an optoelectronic semiconductor device that converts the energy of light incident upon it directly into electricity. The incident light is absorbed in an absorption layer of the solar-cell, which gives rise to the generation of free electrical carriers (i.e., electrons and holes). The free electrical carriers produce a voltage across the terminals of the device, which can be used to directly power electrical systems or be stored in an electrical storage system (e.g., a battery, etc.) for later use.
In order to generate a free-carrier pair in the absorption material, a photon must have energy greater than energy bandgap (EG) of the material. The EG of a material is the energy difference between the top of its valence band (the highest energy level populated by bound electrons) and the bottom of its conduction band (the lowest energy level populated by free electrons). When a photon is absorbed, its energy is given over to a bound electron-hole pair, which frees the electron and enables it to go from the valence band into the conduction band. The energy of a photon is inversely proportional to its wavelength (Ep=hc/λ, where Ep is photon energy, h is Planck's constant, c is the speed of light, and λ is wavelength); therefore, longer-wavelength light (e.g., red light) has lower photon energy than shorter-wavelength light (e.g., blue light). As a result, the choice of semiconductor used to absorb the light has significant impact on the efficiency of a solar cell.
Silicon is perhaps the most commonly used absorbing material in solar-cells at present, due to its relatively low EG, highly developed fabrication infrastructure, and low cost as compared to other semiconductor materials. Unfortunately, silicon does not efficiently absorb light. In addition, since the free electrons and free holes tend to occupy energy levels at the bottom of the conduction band and the top of the valence band, respectively, any extra energy that the electron-hole pairs receive from higher-energy photons is converted into heat that is transferred into the semiconductor material in a process referred to as “thermalization.” Thermalization reduces the energy-conversion efficiency of the solar cell and also raises the temperature of the device, which can lead to degradation and lifetime issues.
In the quest for improved device performance, the solar-cell community has been aggressively searching for material alternatives to silicon, and perovskites are now seen as being among the most attractive silicon substitutes. In fact, in recent years, perovskite-based solar cell efficiencies have become extremely competitive with silicon-based devices. Their rapid rise is the result of a unique combination of properties, including strong optical absorption and long ambipolar diffusion lengths enabled by the benign nature of their intrinsic defects. In addition, perovskites are well suited for use in the top cell of a tandem solar-cell configuration, which enables improved energy-conversion efficiency and thermalization loss than can be achieved in more conventional single-cell devices.
A tandem solar cell is a stacked structure comprising a top photovoltaic portion that is made of a material having a relatively higher EG and a bottom photovoltaic portion that is made of a material having a relatively lower EG. In other words, a tandem solar cell has two p-n junctions and two different band gaps. When light is incident on the stacked structure, high-energy photons are first absorbed in the top portion, while lower-energy photons pass through the top portion to be absorbed in a bottom photovoltaic portion. This enables a broader spectrum of light to be absorbed, thereby improving energy-conversion efficiency beyond the single-junction efficiency limit. In addition, thermalization in the bottom cell is reduced because of the absorption of high-energy photons in the top cell. Depending on the EG of the material of the top solar cell, the fundamental efficiency limit for silicon-based tandem solar cells can be as high as approximately 39%—significantly higher than the theoretical efficiency limit of 33.7% for a single-junction silicon solar cell.
Perovskites are particularly attractive for use in the top cell in tandem solar cell configurations having bottom cells comprising a wide variety of lower-EG materials (e.g., silicon, copper indium gallium selenide (CIGS), etc.) because perovskites have wide, tunable bandgaps and solution processability. As a result, perovskites offer a pathway to achieving industry goals of improving efficiency while continuing to drive down module cost.
Perovskite-based tandem solar cells have been demonstrated in both mechanically stacked, four-terminal configurations and monolithically integrated three-terminal configurations. Mechanically stacked tandem structures have seen the largest success, recently reaching a power conversion efficiency over 24%. The mechanically stacked architecture has some advantages over monolithically integrated structures—most notably, it simplifies device fabrication, allows for silicon surface texturing, and requires no current matching. Monolithically integrated tandem structures, however, have greater promise due to the fact that they have fewer transparent electrode layers.
To date, however, the commercial viability of both single-junction and tandem perovskite-based solar cells has been limited due to thermal and environmental instability issues. Perovskites are susceptible to moisture and methylammonium egress. In addition, halides in the perovskite material can react with metal electrodes, leading to electrode corrosion. Efforts have been made in the prior art to stabilize the perovskite, such as using a hydrophobic heterojunction contact or providing an encapsulation layer that mitigates moisture ingress or a pinhole-free metal oxide layer to prevent metal-halide interaction. Little progress has been made for preventing methylammonium egress, however.
In addition, the top electrode of a solar cell must be highly transparent, as well as highly conductive. Due to fabrication constraints, however, light must first pass through a hole-transport layer (e.g., Spiro-OMeTAD) in a standard architecture, or an electron-acceptor layer (e.g., [6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM), [6,6]-phenyl-C61-butyric acid methyl ester ([61]PCBM), bathocuproine (BCP), etc.) in an inverted architecture, before entering the perovskite, which gives rise to significant parasitic losses.
The de facto industry standard for transparent contacts is a sputtered indium tin oxide (ITO), which is typically deposited as thin film via RF-magnetron sputtering. Unfortunately, the temperatures during deposition and post-annealing may accelerate methylamine evolution, resulting in irreversible damage of the perovskite active layer. In addition, the high energy of the sputtered electrode-material particles can easily damage the perovskite and carrier-extraction layers (i.e., Spiro-OMeTAD, PCBM, bathocuproine (BCP), etc.) during the sputtering process, leading to degradation of device performance.
The addition of a buffer layer on top of the perovskite/carrier-extraction layer stack prior to contact formation is viewed as a potential approach for mitigating the problems associated with ITO deposition. Unfortunately, prior-art buffer layers have poor long-term stability due to their chemical reactivity with perovskite compositions. In addition, prior-art buffer layers have been plagued by low efficiency, which degrades fill factor and open-circuit voltage. Further, many prior-art buffer layers require additional vacuum processes, such as evaporation, which undesirably complicates solar-cell fabrication. Still further, some prior-art buffer layers are not compatible with many desirable device architectures. Molybdenum oxide (MoOx), for example, cannot be used in an inverted solar-cell architecture.
The ability to readily form high-quality, highly transparent contact windows on perovskite-based solar-cell structures remains, as yet, undemonstrated in the prior art.